linux-kernel - Re: [PATCH] ftrace: Access ret_stack->subtime only in the function profiler

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Message-ID: <20160831023500.GB26190@danjae.aot.lge.com>
Date:   Wed, 31 Aug 2016 11:35:00 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH] ftrace: Access ret_stack->subtime only in the function
 profiler

On Tue, Aug 30, 2016 at 10:16:47PM -0400, Steven Rostedt wrote:
> On Wed, 31 Aug 2016 11:11:38 +0900
> Namhyung Kim <namhyung@...nel.org> wrote:
> 
> > On Tue, Aug 30, 2016 at 10:34:41AM +0900, Namhyung Kim wrote:
> > > Hi Steve,
> > > 
> > > On Mon, Aug 29, 2016 at 04:07:00PM -0400, Steven Rostedt wrote:  
> > > > On Mon, 29 Aug 2016 12:05:18 +0900
> > > > Namhyung Kim <namhyung@...nel.org> wrote:
> > > >   
> > > > > The subtime is used only for function profiler with function graph
> > > > > tracer enabled.  Move the definition of subtime under
> > > > > CONFIG_FUNCTION_PROFILER to reduce the memory usage.  Also move the
> > > > > initialization of subtime into the graph entry callback.  
> > > > 
> > > > Hmm, I think documentation needs to be updated. Although it was never
> > > > implemented, I believe I added the subtime to not only work with the
> > > > profiler, but also with the normal tracing (to have the time of the
> > > > internal functions subtracted from the upper level functions). But it
> > > > appears that part was never implemented.
> > > > 
> > > > I'm fine with the patch, or actually implementing what graph-time
> > > > states in Documentation/ftrace.txt. If we take this patch, that comment
> > > > needs to be made to only mention the profiler (and the option should
> > > > only be shown when the profiler is enabled).  
> > > 
> > > Ah, missed the documentation part.  To implement it in the normal
> > > tracing, I think we need to add 'subtime' field to struct
> > > ftrace_graph_ret which will increase disk size.  Are you ok with this?  
> > 
> > On second thought, I think I can do it by just adding value of subtime
> > to ftrace_graph_ret.calltime when graph-time is off.  Then the
> > calltime would not be the timestamp at function entry, but it seems
> > not guaranteed due to the sleep-time anyway.  Now I wonder why it
> > doesn't have 'duration' in the ftrace_graph_ret instead of having
> > calltime and rettime.
> > 
> 
> As it hasn't worked, like forever, I'm thinking of nuking it. Nobody
> seemed to have noticed. I haven't needed to use it, and apparently
> nobody else has either. Why support a feature that nobody uses?
> 
> I have used it for profiling, but not normal function graph tracing.
> You can see the function times inside and do the logic post processing.
> 
> Best bet is to just update the documentation to what the current code
> does.

Ok, will send v2.

Thanks,
Namhyung